1. Field of the Invention
The present invention relates to nylon monofilament yarns for use in industrial fabrics, particular for use in paper machine clothing such as forming fabrics, dryer fabrics or base cloths of press felts.
2. Discussion of Background Information
Paper machine clothing is subject to high abrasive stress due to underside rubbing against support elements and sealing elements such as vacuum boxes, and to the flow through of paper slurry containing abrasive paper fillers. The paper machine clothing fabrics are also subject to bending, stretching and constant flexing in a wet environment. The fabrics are required to keep their strength, dimensional stability, and planarity for as long as possible. Thus it is necessary that the fabric be resistant to abrasion from rubbing against the machine parts and from abrasive fillers in the slurry. It must possess sufficient dimensional stability to maintain sealing against the vacuum boxes and prevent excessive edge wear. The fabric must have adequate machine and cross machine direction strength and stiffness to resist the dynamic load and tension applied to it and the fabric must have sufficient chemical resistance to prevent degradation when subjected to the residual chemicals in the paper slurry and fabric cleaning processes. It has proven very difficult to manufacture a monofilament efficiently that can adequately satisfy these requirements in the fabric. It has proven very difficult to produce a fabric filament material, which is sufficiently resistant to all the stresses a forming fabric is subjected to.
Monofilaments made from polyethylene terephthalate (PET) have been the primary choice for forming fabrics. PET provides sufficient strength, modulus and dynamic stability to resist fabric deformation. It also provides good crimp interchange and the fabric is easily heat set to provide the desired mechanical properties. However, its abrasion resistance is not sufficient for many applications.
To date the most satisfactory solution to the problem of abrasion resistance involves the production of a fabric from polyester, specifically PET in the machine direction, and a blend of 50% polyamide and 50% polyester in the cross machine direction. Nylon (polyamide) 6 (PA6) and nylon 6.6 (PA66) have been the preferred choice for the abrasion resistant filament incorporated into a forming fabric, providing resilience to stresses in all directions. However, they have high moisture regain, which affects the crimp interchange, fabric stability after heat setting and fabric dimensional stability. The poor dimensional stability results in the phenomenon known as ‘edge curl’, which can restrict the application of polyamide filaments in forming fabrics. Furthermore, the reaction of PA6 and PA66 to certain chemicals used in the paper making process and in cleaning fabrics can lead to additional limitations in their applicability.
U.S. Pat. No. 5,169,711 discloses a modified PET material made by melt blending polyethylene terephthalate with a thermoplastic polyurethane (TPU) and extruded into monofilament. The material is claimed to provide adequate abrasion resistance conferred by the TPU phase whilst maintaining those properties provided by PET filaments.
Similar modifications incorporating TPU into other polyesters have been claimed by, for example, Publication WO 93/13251, U.S. Pat. No. 5,700,881, Publication WO 00/56957. All of these modifications by polymer melt blending, prove to be difficult to control, which affect the quality and consistency of the forming fabric filaments. A monofilament made from such a blend provides lower strength as compared to typical forming fabric monofilaments due to the poor compatibility between the two polymers.
Polyamide 612 (PA612) represents a different polyamide material from PA6 or PA66. PA612 typically has lower mechanical strength and stiffness than PA6 or PA66 but it absorbs much less moisture and therefore has better dimensional stability than PA6 or PA66.
However, monofilaments derived from PA612, as supplied, have lower abrasion resistance than the corresponding PA6 and PA66 filaments, and therefore compromise the performance of the fabric.